Peripherals and Storage

Looking at:ScannersPrinters

Why do we need storage devices anyway? What are magnetic disks? How do magnetic disks physically store data? What are optical storage media? How is data stored on optical media? What other options is there for storage?

What Is a Scanner for?

Two main tasks– Converting images (e.g. Drawings and photographs)

into graphical files– As part of an optical character recognition system

converting printed pages into text files

Forms of Scanners

Flat-bed scanner - object is placed over the glass plate. Hand-held scanner - scanner is moved across what

ever is to be scanned

How a Scanner Works

A light source is shines down on to the surface, and is moved along the object being scanned

Light is reflected off the surface of the object Narrow strip of the reflected light hits a row of

detectors (usually a charge coupled device(CCD)) The different levels of light intensity are detected,

converted to numbers, and passed to the computer

Resolution

There are a fixed number of sensors, so image quality is defined by the number of sensors, and measured in pixels (picture elements) the smallest dot that will appear on the scanned image

Quality of a scanned image is usually given in dots per inch (dpi)

Optical Resolution

Optical resolution - the previous slide explained horizontal resolution, and is dependent on the actual detector. There is vertical resolution which is dependent on by how much the scanning head (light and sensors) moves. A vertical resolution of 600dpi means that the head moves 1/600th of an inch each time

The horizontal and vertical resolution both give the optical resolution, e.g 600 x 600 dpi

Interpolated Resolution

This higher than the optical resolution for the same equipment

This performed by software Adjacent pixels are compared and an estimate of what

the pixel would be is produced

Ink-jet Printers

Ink is in a cartridge, which as a circuit board on the outside, which takes signals form the computer to holes in the cartridge. Ink is attracted through the holes that the have a signal set to them. The goes through the holes and sprays onto the paper, forming the required pattern

Colours can be produced by mixing three primary coloured inks

Typical resolution is 600dpi, quality approaches that of a laser printer, at lower cost

Laser Printer

An image is formed on a rotating drum, by a laser in effect drawing an invisible picture on the drum. The laser changes the charge on the drum on the image is formed in this way

Toner (powder) is attracted to where the charge is, and not where there is no charge. Toner is then transferred to the paper from the drum

600dpi or better Capable of high quality images and text More expensive than the other printers

Why do we need storage devices anyway?

We often want to store data or program files that are not in use at a particular time, but we may want to use them later. Memory such as RAM could do this but when the power is turned off what was stored is lost. Therefore, we need storage methods that do not lose information when the power is turned off.

What is common to all forms of storage is they store data.

Magnetic

All magnetic disk storage allows direct access to the data. When data is written to or read from disk, a particular part of the disk can be identified and use by its address.

A disk address uses sector and track. To access a location on disk:

– the read/write heads have to move to the appropriate track– Wait for correct sector to appear under the head.

Disk can usually be read from or write on both the top and bottom of the disk.

The data on a disk is stored in a binary form – as magnetic dots along the tracks, – as one polarity for a 1 and the other polarity for 0.

The read/write head (R/W head) detects the polarity, because there are only two polarities, there are only two states. The pattern of pulses produced by the R/W head in relation to the drive timing is translated into data and set to the processor.

Questions

Does the Read/Write Head touch the disk? No

Does putting a faster processor directly effect the rate of getting data of the disk?

Not directly.

To use the disk has be formatted, the r/w head does under the control of the operating system.

During formatting tracks are created on all the area of the disk used for storage,

a track can be thought of as being concentric bands on the disk, similar idea to tree rings.

Each track into sectors, the closer to the centre of the disk a track is the less sectors per track.

Hard disk have more than one disk stacked vertically, so if data exists on the different platters but at the same head position it is said to be in the same cylinder so all the data in that cylinder can be read without moving the heads.

Because hard disks have the ability to store lots of data with fast access times, at an affordable cost, they are still the main method for storage, holding applications,

data and usually the operating system.

Types of magnetic media

Name three types of magnetic media?

Optical media

Magnetism is not the only way data can be stored, another option is to use optical media.

The disk has holes burnt in it by a laser to record the binary code. A hole or pit on the disk reflects less light than the surface of the disk.

By detecting the difference in reflected light in relation to the time of the drive, a pattern of pulse is produced and this is translated into data sent to the CPU.

Compact Disks

CDs are optical media storing approximately 650MB of data, but are slower than a hard disk to access. The head used for reading from a CD has two parts a laser that shines on a small part of the disk and a light sensor to measure the reflected light.

Digital versatile disk (DVD)

A DVD uses the same basic principle as a CD. the data is packed more densely

– Smaller pit widths and lengths. It has a larger capacity (4.7GB) as compared to a CDs

650MB, it also has a quicker transfer rate than a CD. Compression algorithms help to improve the capacity.

Alternatives

Can you think of any alternatives to those mentioned?

Summary

The main reason we often want to store data or program files that are not in use at a particular time, but we may want to use them later. We need storage methods that do not lose information when the power is turned off.

The disk address uses sector and track. The data on a disk is stored in a binary form as magnetic dots along the tracks

Floppy disk: A floppy disk is a small magnetic disk it is cheap and a floppy drive is still standard on a PC.

Scanner - shining light on to an object,detecting a narrow band of reflected light.

The detector and light source are moved along the object

Quality/resolution of the image is dependent on the number of detectors and is measured in dpi

Interpolated resolution gives a higher resolution than the optical resolutionThree printers - dot matrix, ink-jet and laser

Hard Disk:

– A typical PC has a hard disk with a gigabytes (GB) capacity.

– Because hard disks have the ability to store lots of data with fast access times, at an affordable cost, they are still the main method of storage, holding applications, data and usually the operating system.

Dot matrix- worked by pins pushing ink on to paper in a particular pattern. Quality low but it is cheap

Ink-jet - works by spraying ink on to paper. Reasonable quality

Laser printer- works image being formed on a rotating drum and transferring toner on the paper

Compact Disk (CD)– Optical media where the disk has holes burnt in it

by a laser to record the binary code, and a laser detects the presence of holes and converts it into a binary code.

– storing approximately 650MB of data, – Slower than a hard disk to access.

Digital versatile disk (DVD)– A DVD uses the same basic principle as a CD, – data is packed more densely than CD – Quicker transfer rate than a CD.

Sources for further reading

Chalk et al (2004) pages 144-150 Dick (2002) PC support handbook pages 237 –

249, 260-271, 282-293 and 305-307